1. Field of the Invention
The present invention relates to amplifier circuits and more particularly to an amplifier circuit well suited for amplifying audio signals efficiently.
2. Description of the Prior Art
Output amplifiers heretofore known for amplifying audio signals have been of the type designed for class "B" amplification. Since the class B amplifier has a higher order of efficiency and produces less heat than the class "A" amplifier, it is suitable for use as a high power amplifier. Even in the class B output amplifier, however, the efficiency is on the order of 70 % at the maximum output and this efficiency decreases as the output decreases. Consequently, with a high power output amplifier comprising a class B output amplifier, the amount of heat generated is still great and this makes the design of heat dissipation difficult. With the output amplifier in the integrated circuit version, the maximum output obtainable is limited by the generation of heat.
For example, in the case of an output amplifier comprising a single transistor having its emitter connected to a load resistor, its collector adapted to receive a power supply voltage and its base adapted to receive an input signal, if V.sub.CC represents the power supply voltage and V.sub.in represents the voltage applied to the base, then the value of efficiency .eta. of this output amplifier is given by the following equation: ##EQU1## That is, the efficiency .eta. is improved as the input voltage V.sub.in becomes higher and gets closer to the power supply voltage V.sub.CC.
The output amplifier for audio signals is used in such a manner that even when the voltage of the input signal is at the maximum, it does not exceed the power supply voltage to prevent the distortion in the waveform of the output signal and hence the degeneration of the tone quality. Consequently, the signal level setting is such that the signal of an amplitude which usually appears most frequently has a value considerably lower than the power supply voltage. Thus, the output amplifier of this type is used under conditions that cannot ensure a high efficiency.